So, new belt can't be trained. Back everything up. First, measure from center of tail pulley shaft to center of head pulley shaft. Make sure both sides are equal. This could be a challenge with the electric drive in the way.
Back the head pulley off so that you can see and even turn the tail pulley. Straight edge the bars of the tail, make sure none are bent or folded over. Measure circumference of each side. There can be no difference. ⅛ to ¼ of an inch will aggravate training. Is the tail pulley centered in the conveyor frame?
Adjust head pulley take up so that the belt has droop on the bottom and has just enough tension to not slip.
Check that your belt skirting is not set too low causing a drag on the belt.
If the tail is carrying the belt to the side and you've pitched the return rollers 100% in the opposite direction, it really looks like you have a damaged tail pulley.
Sometimes in transport when they split the screen box and underscreen conveyor, they knock stuff around or tie it down too tightly instead of blocking it up.